The disclosure is directed to doped, silica-titania ultra-low expansion (“ULE”) glass, glass articles and methods for making the same.
Extreme Ultra-Violet Lithography (“EUVL”) is an emerging/leading lithography technology for the 13 nm node and beyond for manufacturing Micro Processing Units (“MPU”) and Dynamic Random Access Memory (“DRAM”) chips. The EUVL scanners which are used to produce MPU, DRAM and other integrated chips have been produced in small numbers to demonstrate this technology. Projection optics systems, particularly reflective optics systems, are an important part of these scanners. Low thermal expansion glass, for example Corning® Incorporated ULE® glass, has been used for making the projection optics systems used in EUVL scanners. Advantages of a low thermal expansion glass such as ULE® Glass include: polishability to the required finish, coefficient of thermal expansion (“CTE”) and expansivity control, and dimensional stability.
As the development of EUVL systems proceeds, the specifications are becoming more stringent for newer optics systems, particularly those employed in high-volume integrated chip and semiconductor device manufacturing. As a result, the specifications for the materials employed in these optics systems, including CTE and expansivity criteria, are becoming increasingly more difficult to achieve.
More particularly, the mirror substrates employed in the projection optics systems within EUVL scanners must meet stringent CTE requirements in order to maintain their intended surface shape upon temperature changes caused by normal operation cycles of the scanners. As the EUVL source power increases to meet the requirements of a high volume manufacturing system, the specifications for the projection optics mirror substrates are becoming even more stringent. Current glass compositions employed in these substrates have approached a rate of change of CTE with temperature (also known as “CTE slope” or “expansivity slope” with units of ppb/K2) of about 1.35 ppb/K2 at 20° C. However, newer specifications for mirror substrates are desired that would require CTE slope levels of less than 1.35 ppb/K2, and for some applications CTE slope levels as low as, or even lower than, 0.6 ppb/K2 at 20° C. will be required.
Accordingly, there is a need for ULE glass, and methods of making it, that can be employed in EUVL applications with low expansivity and reduced expansivity slope and, even more preferably, low expansivity and reduced expansivity slope with a high level of polishability.